1. Field of the Invention
The present invention relates to a glass substrate for magnetic recording media, a process for producing the same, and a magnetic recording medium obtained with the substrate. More particularly, the invention relates to a glass substrate capable of giving a magnetic recording medium which is effective in attaining a reduced magnetic-head flying height and is so reliable that the information magnetically recorded therein is not lost or attenuated even upon long-term use at a high rotational speed. The invention further relates to a process for producing this substrate.
2. Description of the Related Art
With remarkable progress in the handling of digital information in recent years, various devices for storing such information therein have been developed. Improvements in these devices are ever-progressing, and the information recording capacity and the speed of recording/reproducing are increasing at an annual rate of tens of percents. In particular, the information recording devices which are currently used most widely are magnetic disks, and they are being improved at a higher rate than the other devices.
Under these circumstances, magnetic disks are required to be magnetic recording media capable of recording information at a higher density. For satisfying this requirement, the substrates for magnetic recording media also have come to be required to have high flatness, high smoothness, and rigidity. Because of this, glass substrates, which can be easily ground and polished, are recently coming to be employed as a substitute for the aluminum substrates, which have conventionally been used mainly.
The aluminum substrates for use in this application usually have a multilayer constitution comprising aluminum substrate/nickel layer/phosphorus layer. In the surface of the phosphorus layer, an anisotropic texture comprising concentric marks (an oriented surface roughness) is formed by a mechanical means such as grinding. A magnetic film formed on such an anisotropic texture has C-axis orientation in a circumferential direction, which is caused by an anisotropic film stress, to constitute a so-called oriented medium. This kind of substrate is disclosed in JP-A-6-231442. (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d.)
On the other hand, the magnetic recording media employing a glass substrate which have been practically used are ones having an isotropic surface roughness having no anisotropic texture (the term xe2x80x9cisotropic surface roughnessxe2x80x9d is herein used also for magnetic recording media having a surface roughness formed at random). However, the recording media having an isotropic surface roughness have been found to have the following drawback. When such a recording medium is one produced so as to be capable of high-density recording, there are cases where signals once written therein are lost or rapidly attenuated, making it impossible to conduct highly reliable magnetic recording.
In JP-A-63-160010 is disclosed a glass substrate for magnetic recording media obtained by forming a texture in a circumferential direction in a smooth surface of a glass substrate by a mechanical method or chemical etching. However, the magnetic recording medium employing the substrate obtained by this process has a problem that it cannot have a fine surface roughness and, hence, it is difficult to glide a magnetic head thereover at a small height (to scan the magnetic head with a low glide height) so as to enable high-density recording. It has hence been proposed recently to use a mixture of cerium oxide, which has the high ability to abrade glasses, and a diamonds lurry (see JP-A-2000-101656). Other techniques which have been proposed include to use a slurry containing a solution having hydroxyl groups, such as an aqueous potassium hydroxide solution or aqueous sodium hydroxide solution, and thereby impart a chemical action to a mechanical processing force (see JP-A-2000-301441 and JP-A-2001-9694).
The technique of mechanically imparting a surface roughness (mechanical texture) to the multilayered film composed of a nickel layer and a phosphorus layer has a drawback that the processing for forming the roughness tends to generate foreign particles or dust particles and this not only results in a reduced yield in the production of magnetic recording media but also leads to an increased cost. It has hence been proposed in the reference cited above to use a substrate obtained by directly forming an anisotropic texture on a surface of a glass substrate. However, this technique has a drawback that since glass plates have a higher surface hardness than aluminum substrates, it is difficult to use the above-described technique of the related art to form a fine texture. Furthermore, the technique of using-cerium oxide, which has the high ability to abrade glasses, and the technique of using a slurry containing a solution having hydroxyl groups have the following drawback. Although these techniques are effective in forming a fine texture, the shape of the texture readily changes upon cleaning with a chemical due to the high fineness thereof. Because of this, there have been severe restrictions on the use of chemicals in removing diamond or other abrasive grains caught in grooves of the texture. In particular, acid aqueous solutions, which are highly effective in cleaning multicomponent glasses, cannot be used because they considerably change the texture shape. Due to such restrictions, substrates obtained through the texturing treatment often have residual abrasive grains which were contained in the slurry used.
An object of the invention is to eliminate the problems described above, and, more specifically, is to provide a glass substrate giving a magnetic recording medium in which reading/writing operations can be conducted while gliding the magnetic head at a smaller height, and to provide a process for producing the glass substrate.
According to a first aspect of the invention so as to solve the above problems, a process for producing a glass substrate for magnetic recording media is provided which comprises forming a surface roughness in the main surface of a glass plate which has been formed into a disk shape, wherein the surface roughness is formed by imparting processing marks having a permanent strain to the main surface of the glass plate in a circumferential direction thereof and then chemically etching the whole main surface of the glass plate.
The term xe2x80x9cprocessing marksxe2x80x9d as used herein means those surface and near-surface parts in a glass which have a residual permanent strain formed by a mechanical stress without or with a physical change in shape. Although it is usually difficult to directly subject a glass substrate, which has a high hardness, to texturing (surface processing) by a mechanical means, a permanent strain can be formed therein relatively easily.
In one preferred embodiment of the process of the invention for producing a glass substrate for magnetic recording media, the surface roughness is formed based on a difference in the depth of chemical etching between the areas having the processing marks and the areas not having the processing marks. This method of surface roughness formation according to the invention is based on the fact that the glass in the areas having a permanent strain differs in resistance to acids or alkalis from that in the areas not having a permanent strain. Specifically, the surface roughness formation is based on the fact that the areas having a permanent strain have higher acid resistance (lower susceptibility to chemical etching with an acid solution) than the areas not having a permanent strain, or is based on the fact that the areas having a permanent strain have higher alkali resistance (lower susceptibility to chemical etching with an alkaline solution) than the areas not having a permanent strain.
The fact described above was experimentally found by the present inventor. Based on this fact, processing marks having a permanent strain in a circumferential direction of the glass substrate are imparted to a surface of a glass and to near-surface parts thereof, and the whole glass surface is then chemically etched, for example, with a solution of an acid or alkali. In this etching, the areas bearing the processing marks having a permanent strain have a lower etching rate in the glass depth direction than the areas having no or a smaller amount of permanent strain. Because of this, the areas having a permanent strain leave protrusions after the etching. Thus, a surface roughness (texture) which is constituted of the oriented protrusions in a circumferential direction of the glass substrate and is useful in magnetic recording media can be formed.
The impartation of processing marks to a disk-form glass plate in a circumferential direction thereof can be accomplished by a method in which a substrate or film having fixed thereto fine particles of a material harder than the glass is pressed against the surface of the glass plate kept being rotated. Alternatively, the impartation may be accomplished by a method in which a substrate or film is kept being pressed against the glass plate kept being rotated, and hard fine particles are supplied to between the glass plate and the substrate or film.
In another preferred embodiment of the process of the invention for producing a glass substrate for magnetic recording media, the processing marks are formed by rubbing the main surface of the glass plate with a processing tape while supplying a processing fluid comprising a slurry to the main surface of the glass plate.
A suitable method for forming processing marks in a circumferential direction of the main surface of the glass plate is to rub the surface of the glass plate in a circumferential direction with a processing tape and a processing fluid comprising a slurry. For example, it is preferred to use a method in which a processing tape is pressed against the glass plate surface while supplying the processing fluid to the glass surface to thereby press fine particles suspended in the processing fluid against the glass plate surface. By this method, the formation of processing marks in the circumferential direction can be satisfactorily controlled.
The material of the processing tape to be used is not particularly limited, and known processing tapes can be used. Examples thereof include resin tapes made of a polyester, cellulose, nylon, or the like. The slurry may be one comprising a liquid, e.g., water, and an abrasive material suspended therein. A suitable kind of abrasive material is selected according to the desired texture to be formed. Although an abrasive material can be selected according to the desired texture shape, smaller abrasive materials usually give finer textures.
In still another preferred embodiment of the process of the invention for producing a glass substrate for magnetic recording media, the chemical etching is conducted with an etchant containing either hydrofluoric acid or hydrosilicofluoric acid or containing a mixture of these acids.
For the chemical etching after the formation of processing marks, it is preferred to use an etchant with which the shape of the texture can be suitably controlled. An example of such an etchant is one containing hydrofluoric acid or hydrosilicofluoric acid. This etchant is preferred in that a sufficiently large difference in etching rate between the areas having a permanent strain and areas not having a permanent strain can be obtained and that the amount of glass to be removed by etching can be easily controlled. An alkaline solution also can be used as an etchant in the invention.
In still another preferred embodiment of the process of the invention for producing a glass substrate for magnetic recording media, the glass substrate which has undergone the roughness-forming treatment is subjected to a chemical strengthening treatment in which part of the ions contained in the glass substrate are replaced with ions contained in a molten salt which have a larger ionic radius than those ions.
After the chemical strengthening treatment, cleaning may be conducted as a final cleaning step for satisfying the cleanliness required of glass substrates for magnetic recording media. This step is conducted in a highly controlled clean room. In contrast, the roughening treatment described above cannot be conducted in a clean room due to the necessity of using a slurry. Consequently, when a roughening treatment is to be conducted after the chemical strengthening treatment, the process is complicated, for example, because it is necessary to install a separate booth in the clean room.
According to this embodiment, the production process is simple and the production cost can be reduced because a chemical strengthening treatment is conducted after the roughening treatment.
In still another preferred embodiment of the process of the invention for producing a glass substrate for magnetic recording media, the glass substrate which has undergone the chemical strengthening treatment is subjected to cleaning with an acid aqueous solution and with an alkaline aqueous solution successively.
The acid cleaning and alkali cleaning, which are conducted after the chemical strengthening treatment, are intended to regulate the shape of the texture (roughness) which has been altered by the chemical strengthening treatment. The mechanism of this cleaning is as follows. There are cases where the processing marks formed in a circumferential direction of the glass substrate relax and expand due to heat during the chemical strengthening and the ridges of the resultant texture have an excessively large height. On the other hand, when this glass is cleaned successively with an acid and an alkali, part of the glass components selectively dissolves in the acid to leave a framework layer containing silica as the main component.
This framework layer is porous and, hence, undergoes mild etching with the alkali. Thus, by conducting acid cleaning and alkali cleaning after the chemical strengthening treatment, the shape of the texture ridges which have been made excessively high can be regulated. Since relaxation of the permanent strain has progressed after the chemical strengthening treatment, a large difference in etching rate such as that described above does not arise and, hence, the cleaning does not result in further texture growth. Furthermore, since the permanent strain has not relaxed completely, the texture does not disappear through the etching, unlike textures formed by a mechanical method alone. Because of these, the chemical strengthening treatment is not accompanied by a considerable change in shape. The cleaning with an acid and an alkali functions also to remove impurities such as the strengthening salt and iron which adhered during the step of chemical strengthening treatment.
In still another preferred embodiment of the process of the invention for producing a glass substrate for magnetic recording media, a glass substrate for magnetic recording media produced by the process of the invention, wherein the main surface thereof has a surface roughness of from 0.5 to 1.0 nm in terms of Ra value as determined with an AEM, and the value of Rmax obtained by subtracting the minimum height of the roughness from the maximum height thereof is 3.0 nm or higher.
Values of Ra lower than 0.5 nm are undesirable in that the magnetic recording medium has an increased area of contact with a magnetic head, so that the magnetic head is apt to stick to the recording medium, resulting in a heightened probability of rotation troubles. On the other hand, values of Ra exceeding 1.0 nm are undesirable in that the magnetic recording medium has too small an area of contact with a magnetic head, and that when abnormal projections which are only slightly too high are present, they may collide with a magnetic head and abrade, often resulting in head crushing or head corrosion. The value of Rmax is preferably 3.0 nm or higher from the standpoint of preventing a magnetic head from sticking to the surface of the magnetic recording medium.
In still another preferred embodiment of the glass substrate for magnetic recording media of the invention, the value of Rmax is 15 nm or lower. This is because values of Rmax exceeding 15 nm result in a further heightened probability of head crushing.
In still another preferred embodiment of the glass substrate for magnetic recording media of the invention, a glass substrate for a magnetic recording medium is produced by the process of the invention, wherein the main surface of the glass substrate has a surface roughness of 0.2 to 0.5 nm in terms of Ra value as determined with an AFM, and the value of Rmax obtained by subtracting the minimum height of the roughness from the maximum height thereof is 2.0 nm or higher. The value of Rmax is preferably not higher than 15 nm because values of Rmax exceeding 15 nm result in a further heightened probability of head crushing.
The substrate according to this aspect is applicable also to hard-disk drives of a type of recording employing a small head flying height, such as the ramp road type.
In another preferred embodiment of the glass substrate for magnetic recording media of the invention, the roughness formed in the main surface of the glass substrate is linearly arranged in a circumferential direction of the substrate so that the main surface has a radial-direction line density of 5,000 to 40,000 lines per mm as determined with an AFM.
In another preferred embodiment of the glass substrate for magnetic recording media of the invention, a magnetic recording medium comprises the glass substrate of the invention described above and a magnetic recording film including a magnetic film, so that the magnetic recording film covers the main surface of the glass substrate.
Since the magnetic recording medium of the invention has a magnetic recording film covering the glass substrate having a surface roughness in a circumferential direction, the magnetic film can have such magnetic anisotropy that the coercive force thereof in the circumferential directions is not lower than that in the radial directions.